With 83 percent of Americans saying they disapprove of the way Congress is doing its job, one might expect presidential candidates and their savvy political teams to keep themselves at a distance.

But even in this year of the outsider — where, from the revolt in the House to the polls in the presidential race, it might appear the strength of the establishment is on the downturn — candidates seeking the highest office in the land are competing for their endorsements in what has been described as the “Invisible Primary .” On Friday, Sen. Bernard Sanders, the Vermont independent giving establishment favorite Hillary Rodham Clinton her biggest challenge for the Democratic nomination, is expected to receive an endorsement from Arizona Rep. Raúl M. Grijalva during a campaign rally in Tuscon.

Grijalva would be the only congressional lawmaker to endorse Sanders for president so far — a figure dwarfed by the more than 140 members of Congress who have said they are ready for Hillary.

“If I were Bernie Sanders, I’d be out bragging about that,” said Stuart Stevens, a senior strategist for Republican presidential candidate Mitt Romney’s 2012 campaign. “Look at congressional approval. It fits his whole rationale for his candidacy.”

Endorsements from members of Congress are an inherent reflection of establishment support, Lynn Vavreck , a professor of political science at the University of California, Los Angeles, wrote for The New York Times' Upshot blog. But, she added, research suggests that support from political elites is also "the single best predictor" of a candidate's success.

"It’s a signal about competence and viability generally," she told CQ Roll Call.

Clinton leads that race on the Democratic side, where endorsements from members of Congress have real practical importance due to their status as super delegates — not pledged to any candidate in the party's presidential nomination process. Other than that, Sen. Claire McCaskill, a Missouri Democrat who was the first in the Senate to endorse Clinton this time around, said she did not see too much value in them.

"The people who think endorsements are important are all in Washington," she said.

On the Republican side, "sometimes they can be counterproductive," McCaskill added, "because the base of their party doesn’t want anything to do with them because they’re an elected official."

In fact, the candidate with perhaps the closest ties to the party’s establishment, Jeb Bush — the former Florida governor who is trying to get his family a third seat at the presidents club — has earned the most congressional endorsements, including 20 members of the House and three senators.

Even though that support is not reflected in the polls, Bush's campaign said it expects it to pay off in the long run.

“Organizational support is important in a campaign,” said Tim Miller, a spokesman for Bush’s campaign. “We are a building a campaign that can last the long nomination fight, getting support of elected officials at the local, state, and federal level is an important part of that effort.”

Several Republican presidential operatives interviewed said the effectiveness of congressional endorsements really depends on who is endorsing.

“I’ve always felt that endorsements only matter to the degree that they help you deliver a message,” Stevens said. “If they are delivering a message and they are a validated messenger, then I think it becomes important.”

In other words, it is one thing for an endorser to sign on to a press release. It is another for them to hit the campaign trail for the candidate they support.

Arizona Sen. John McCain — a two-time presidential candidate who was the party’s nominee in 2008 — has done the latter, appearing dozens of times in Iowa and New Hampshire with South Carolina Sen. Lindsey Graham. McCain is the only member of Congress who is supporting Graham's long-shot bid, but those close to Graham say his assistance on the trail has attracted attention he may not have otherwise received because of his position low in the polls.

“It gives Sen. Graham some extra credibility,” said Jon Seaton, a senior adviser to his campaign. "There’s a very special relationship between John McCain and New Hampshire voters.”

In terms of messaging, McCaskill said endorsers can do one big thing: play the role of the attack dog.

"Sometimes the campaign is not the most effective mouthpiece for countering unfair attacks or confronting distortions," she said. "Sometimes it’s better to have someone who is separate from the campaign do that."

While many Republican members of Congress are still on the sidelines (the number of endorsements in the presidential primary is at one of the lowest points in the past six elections , second only to 2012), outsider favorites such as Donald Trump and Ben Carson both have exactly zero congressional endorsements.

“I think Donald Trump is an example of someone endorsements would help," Stevens said. "If the governor in one of those primary states came out, I think it would help him a lot.”